This invention relates to a method and an apparatus for cleaning gases in order to prevent contamination of substrate surfaces within localized spaces, particularly for preventing the contamination of substrate surfaces such as those starting materials, semi-finished products and finished products in areas of advanced technology such as semiconductor and liquid crystal fabrication.
The method and apparatus of the invention for cleaning gases are applicable to the following areas: spaces within cleanrooms as in semiconductor fabrication plants, liquid crystal fabrication plants and precision machine manufacturing plants, clean boxes, valuables storages, wafer stockers (wafer boxes), closed spaces for the transport of valuables, clean closed spaces and transport spaces in the presence of various gases or under reduced pressure or in vacuo, spaces containing gases to be supplied to cleaning apparatus, and spaces for access to apparatus for supplying air to create an air knife, as well as to interface apparatus.
Two kinds of contaminants are of concern to cleanrooms in semiconductor fabrication plants and they are fine particles (particulates) and gaseous substances such as non-methane as hydrocarbons (HC) that are present in air at extremely low concentrations due to automotive exhaust emissions and the outgassing of various resin products widely used in consumer applications.
The filters installed in cleanrooms are incapable of HC removal, so the HCs in the external air will be brought into the cleanrooms. However, extremely low concentrations of HCs in normal air (both within and outside the room) can be a contaminating harmful gaseous substance and hence need be removed. Recently, the outgassing of high-molecular weight resins which are constituent material of cleanrooms is also a problematic source of HC generation. Since the cost performance of the present-day large-scale cleanrooms is relatively low, the future cleaning technology will see it necessary that localized spaces (control spaces at a reduced scale) should be cleaned with high efficiency. However, high-molecular weight materials such as plastics, sealants and adhesives will be used in greater amounts in localized spaces than in the present-day cleanrooms and the outgassing of HCs from these high-molecular weight materials will be a contamination source of concern. The various solvents (e.g. alcohols and ketones) used during operations in cleanrooms evolve HCs, which are also contaminants of concern at increased concentrations.
Thus, two sources exist for the generation of HCs in cleanrooms and the HCs brought into the room from the external air are combined with the HCs generated within the room, whereby the concentration of HCs in the cleanroom is sufficiently higher than that in the external air to contaminate substrate surfaces.
If fine particles (particulates) deposit on the substrate surfaces of wafers, semi-finished products or finished products, interconnections in the conductor patterns may break or shorting may occur to produce defects in the circuit. The HC deposit on the substrate can also reduce its compatibility for the resist, causing unevenness in the thickness of the resist film or deterioration in its adhesion to be substrate. Therefore, the HCs as contaminants will reduce the production rate (yield) of semiconductor devices. In the latest type semiconductor fabrication plants, the rate of air circulation within cleanrooms is increased for the purpose of energy saving and this causes the HCs in the cleanroom to be concentrated to much higher levels than in the external air; as a result, the HCs will deposit on semiconductor substrates to contaminate their surfaces.
The degree of this surface contamination can be expressed by contact angle on substrates and severe contamination produces a large contact angle. Substrates with large contact angles permit film formation but the adhesion of the film is so weak that the yield of fabrication will be reduced. The term "contact angle" as used herein refers to the angle of contact by wetting with water and represents the degree of contamination of the substrate surface. If a hydrophobic (oily) substance deposits on the substrate, its surface repels water and will not be easily wetted. Then, the angle of contact between the substrate surface and a waterdrop will increase. In other words, a large contact angle means a high degree of contamination and vice versa.
Conventional methods of purifying the air in cleanrooms comprise (1) mechanical filtration (with, for example, HEPA filters) and (2) electrical filtration by electrostatic trapping of fine particles (particulates) with charged or electroconductive filters (e.g. HESA filters) under high voltage. Either method is intended for removing fine particles (particulates) and ineffective for removing gaseous contaminants such as non-methane HCs which will increase the contact angle.
Known methods for removing HCs as gaseous contaminants include pyrolysis and ozonolysis but none of these methods are effective for removing HCs that are present at extremely low concentrations in the air introduced into cleanrooms.
The present inventors already proposed methods and apparatus for preventing the contamination of substrate surfaces by means of adsorbents or absorbers which were capable of preventing the increase in contact angle (Japanese Patent Public Disclosure Nos. 157284/1993, 324/1994 and 8752/1995). These proposals were found to have the problem of waste generation from the adsorbents or absorbers used to remove HCs, so the inventors proposed a method and an apparatus characterized by the use of dust removing means and means of decomposing hydrocarbons with the aid of photocatalysts (Japanese Patent Public Disclosure No. 57981/1995). The proposal is effective in certain application areas but needs further improvements in order to enhance its practical feasibility in cleanrooms.